[MINOR][CORE] Fixed doc for Accumulator2.add

## What changes were proposed in this pull request?

Scala doc used outdated ```+=```.  Replaced with ```add```.

## How was this patch tested?

N/A

Author: Joseph K. Bradley <[email protected]>

Closes apache#13346 from jkbradley/accum-doc.

Author: jkbradley

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1304,7 +1304,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   }
 
   /**
-   * Create and register a long accumulator, which starts with 0 and accumulates inputs by `+=`.
+   * Create and register a long accumulator, which starts with 0 and accumulates inputs by `add`.
    */
   def longAccumulator: LongAccumulator = {
     val acc = new LongAccumulator
@@ -1313,7 +1313,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   }
 
   /**
-   * Create and register a long accumulator, which starts with 0 and accumulates inputs by `+=`.
+   * Create and register a long accumulator, which starts with 0 and accumulates inputs by `add`.
    */
   def longAccumulator(name: String): LongAccumulator = {
     val acc = new LongAccumulator
@@ -1322,7 +1322,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   }
 
   /**
-   * Create and register a double accumulator, which starts with 0 and accumulates inputs by `+=`.
+   * Create and register a double accumulator, which starts with 0 and accumulates inputs by `add`.
    */
   def doubleAccumulator: DoubleAccumulator = {
     val acc = new DoubleAccumulator
@@ -1331,7 +1331,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   }
 
   /**
-   * Create and register a double accumulator, which starts with 0 and accumulates inputs by `+=`.
+   * Create and register a double accumulator, which starts with 0 and accumulates inputs by `add`.
    */
   def doubleAccumulator(name: String): DoubleAccumulator = {
     val acc = new DoubleAccumulator